The present invention relates to high performance sport boats with a length of about twenty-five feet or less. More particularly, the invention relates to marine hydroplaning foils for high performance sport boats having a propeller drive.
High performance sport boats such as drag racing boats, ski boats and competitive bass boats having outboard propeller drives are often equipped with a device that is characterized variously as an anti-cavitation plate, a hyrofoil, a planing foil or a stem lifting plate. Although different functions are implied by each of these descriptive characterizations, each represents a certain measure of descriptive accuracy.
Generally, an anti-cavitation plate is a relatively thin, fin-like element that radiates in a horizontal plane from the propeller drive shaft housing in close proximity above the propeller arc. If the plate structure is present by design of the propulsion apparatus manufacturer, the plate device is usually an integrally cast component of the drive shaft housing. If applied to the propulsion apparatus as an auxiliary or accessory device, the plate structure may be externally secured to the housing by welding or by brackets that are attached by threaded fasteners.
At low boat speeds when the boat propeller and a cooperative anti-cavitation plate are completely submerged, water flowing against the plate is deflected downward by the plate bottom surface into the propeller to increase the hydrodynamic pressure around the propeller. Such increased fluid pressure around the propeller retards the onset of propeller blade cavitation. Blade cavitation is a hydrodynamic condition whereat a propeller blade is moving at a velocity greater than the surrounding fluid medium can flow into the space displaced by the blade. Hence, the propeller blade path leaves a vacuum behind the blade which retards the blade advance and increases the magnitude of power required to maintain the blade speed. Since the plate device above the propeller tends to increase the speed at which cavitation begins, the "anti-cavitation" description is valid.
From a different perspective, the fluid ram forces that generate greater fluid pressure below the plate also generate lift and upward force on the plate structure. Since the plate structure is secured to the drive shaft housing and, ultimately to the boat transom, this lifting force on the plate effectively lifts the boat stem to a more shallow depth of penetration below the water surface.
Competitive speed boats, for example, take advantage of the anti-cavitation and lifting synergism with foil plates such as those known in the art as Whaletail.RTM. or StingRay.RTM.. Proprietary foil plates such as these increase the plate area to improve the boat "hole-shot" performance, i.e., the elapsed time from a dead-stop in the water to top-surface planing. As an average, a good hole-shot time is about 5 seconds. Hence, it is also valid to characterize the device as a "stem lifting plate".
Boats of the type and capacity to which the present invention pertains have speed capabilities considerably in excess of 60 mph. Although a stem lifting plate is useful for quickly achieving that speed, the utility ends when the plate rises above the water surface. At 80 m.p.h. and above, half or less of the propeller circle engages the water surface and a stem lifting plate is entirely above the water surface. The boat is supported at the stem end on a small area of the keel/bottom area. The boat bow is supported by aerodynamic ground effect. When a boat is running in this configuration, a prior art anti-cavitation plate or stem lifting plate becomes the source of speed retarding drag.
Above 80 m.p.h., about half or more of the propeller circle is above the water surface. As a propeller blade turns forcefully from the gaseous atmosphere of the air above the water surface into the liquid surface, the impact produces a forward and upwardly directed spray from the water surface. Similarly, as a propeller blade rises from the liquid body, water attached to the blade surface is propelled centrifugally into the atmosphere above the water.
In sum, a stem lifting plate that provides a substantial planform area above a propeller blade may greatly improve the boat hole-shot with a rapid rise of the unit out of the water and onto the planing step. However, once on the planing "step", this same structure above the water surface tends to reduce the boat top or maximum speed. Therefore, to gain one or two seconds from the hole-shot, a competitor may sacrifice ten or more seconds in his overall time due to high speed parasitic forces on the lifting plate. Descriptively, the prior art lifting plate receives and deflects the propeller spray back against the propeller into the water surface proceeding the propeller. Water that is hurled from the propeller blade by centrifugal force strikes a prior art stem lifting plate in a spray of droplets. This highly energized spray strikes the prior art foil in an area aft of the propeller plane. However, the longitudinal pitch axis of the boat under this operating condition is forward of the propeller plane. Consequently, the impulse force of the propeller spray striking the prior art lifting plate behind the propeller plane induces a force couple that presses the boat bow downward against the aerodynamic lift and into the water surface. Due to increased water surface contact by the boat bow, drag forces on the boat are increased thereby reducing the boat speed. Furthermore, because the propeller spray is deflected by the prior art foil back into the spray jet from the propeller, reactive thrust from the propeller is reduced by the jet disturbance.
It is an object of the present invention to provide a hydro-planing foil that does not sacrifice the top speed capacity of a boat for a short-term hole-shot acceleration period.
A further object of the present invention, therefore, is to provide a hydro-planing foil having minimal surface area within the propeller plane to interrupt the spray of water from the propeller.
Another object of the invention is a hydroplaning foil having a trailing edge that is advanced from the propeller rotational plane.
Also, an object of the invention is a hydroplaning foil having down turned lateral tips to augment lift of the vessel out of the water when starting and to controllably shed propeller water spray away from the propeller arc.